Fuel cells convert chemical energy in the form of chemical bond potential to electrical energy in the form of electric current. Fuel cells replace batteries in many applications, typically due to a need for a higher energy density or higher discharge rate.
Like a battery, a fuel cell converts electrochemical energy into electrical current and may use liquid electrolytes between electrodes. Like a battery, a typical fuel cell supports oxidation and reduction, but unlike a battery, they occur on opposite sides of a "selectively-permeable" barrier separating electrodes. The barrier passes only selected species, such as oxygen ions, between its two sides so that oxidation and reduction may both proceed at opposite electrodes. Like a battery, the fuel cell passes electrons released at the oxidation electrode back to the reduction electrode. The resulting current flow can be directed to an electrical load attached to the electrodes by leads.
Solid oxide fuel cells (SOFC) are a relatively recent innovation in which two porous electrodes, bonded to a solid oxide ceramic between them, form the selectively-permeable barrier. On one side of the barrier is a fuel, on the other side oxygen. Most reactants cannot pass through the barrier, but oxygen ions can flow through the solid oxide lattice. The electrodes are typically formed of electrically conductive metallic or semiconducting ceramic powders, plates or sheets which are porous to oxygen ions from the cathodic, oxygen side of the barrier.
Suitable fuels include hydrogen and simple hydrocarbons. Hydrogen was an early favorite fuels but is problematic for generation, storage, handling, and resultant cost. Simple hydrocarbons, such as methane (CH.sub.4), can be used, but are preferably reformed into simpler reactants prior to entering the fuel cell to be efficiently oxidized in the fuel cell. U.S. Pat. No. 4,910,100 (Nakanishi et al.) issued Mar. 20, 1990 describes the reformation process.
As an outgrowth of the aerospace industry of recent decades, fuel cells are well documented in the art along with the preheating and reformation processes which prepare fuels for rapid reaction in solid oxide fuel cells. For example, U.S. Pat. No. 4,910,100 (Nakanishi et. al) issued Mar. 20, 1990 discusses the four most common types of fuel cells, their electrolytes and chemical reactions, and ways to increase the efficiency of reactions. Nakanishi et al. also discuss the temperatures of reformation and a method for extracting the heat of reaction to drive the reformation process within a fuel cell.
U.S. Pat. No. 4,876,163 (Reichner), issued Oct. 24, 1989, discloses various interconnected, cylindrical fuel cells arranged to uniformly distribute the temperature. U.S. Pat. No. 4,721,556 (Hsu) issued Jan. 26, 1988, discloses a stack of interconnected solid oxide fuel cells. U.S. Pat. No. 4,943,494 (Riley), issued Jul. 24, 1990, discloses a system of integrated fuel, air and exhaust conduits which support the operation of fuel cells in a compact arrangement. U.S. Pat. No. 4,983,471 (Reichner et al.), issued Jan. 8, 1991, discloses a fuel cell arrangement in which reformable fuel is exposed to a catalyst just prior to reaction in the solid oxide fuel cells. Likewise, U.S. Pat. No. 5,079,105 (Bossel), issued Jan. 7, 1992, discloses a solid oxide fuel cell in which an endothermic reformation process occurs inside the center of a stack of fuel cells as the fuel is distributed.
In the art, packed beds of granular or pelletized material having a catalyst as a surface coating are known. The packed bed may be called a reactor, reactor bed, or the like. The use of such packed beds to serve as reformers is known. The beds are effective because of the long tortuous path that exposes reactants to a large surface area of catalyst during an extended dwell time during passage through the bed.
A stack of solid oxide fuel cells operating at temperatures over 1000.degree. C. is contained within a chamber called a stack furnace. The stack is an assembly of several fuel cells assembled in close proximity, typically sharing intermediate walls. Since the endothermic reaction of reforming a feedstock into a suitable fuel requires almost a third of the exothermic heat of combustion, heat has been recovered from stack furnaces to drive the associated reformation reactions. Transferring the recovered heat from a stack furnace to an external (remote) reformer often implies unacceptable bulk, heat losses, and temperature losses.
Some fuel cells dispense with the reformer, relying instead on anodic reformation in the stack of fuel cells itself. Anodic reformation is a process by which feedstock simply arrives in a fuel cell and reforms at the anode before oxidizing them to produce electrical current. Obviously, the presence of so many intermediate reactions and species at the anode creates an access problem. An "atomic or molecular traffic jam" occurs as atoms try to reach proper reaction sites with the requisite energy. All reactions slow. Of course, the rate-limiting reaction in the chain of reactions occurring during the process may be as severely affected as any other. The resulting inefficiency of this overall process detracts from the mechanical simplicity of the scheme.
Internal reformers placed directly inside the stack furnace recover the heat and temperature losses experienced by remote reformers without burdening the fuel cells' anodes with the reformation duty. However, such placement is not without problems. For example, in the temperature range of reformers in the stack furnace, the formation of elemental carbon is rapid, even overwhelming.
The cracking process, which dissociates the constituent atoms of carbon and hydrogen from methane, is temperature sensitive. At the same time, little energy is required to break the interatomic bonds. Thus, the cracking process converting methane to elemental carbon can occur very rapidly with minimal energy so long as a high temperature is maintained in the feedstock. In an internal reformer, such a condition exists inherently.
Carbon coats and damages catalytic surfaces and clogs interstices within the reactor bed of the reformer. Agglomerated elemental carbon does not react as readily as individual atoms in the flow through the reformer. Therefore, both effects tend to be irreversible, rendering much of the catalytic surface inaccessible, useless or both.
The reformation process, properly controlled, is a balancing act to keep the multiple reactions operating at a point close to thermodynamic equilibrium. No individual reaction, such as carbon formation, should be permitted to get out of control. Thermodynamic equilibrium is maintained by controlling the species present, the temperature of the flow, and the heat flux into the flow. The need for thermodynamic equilibrium is largely dependent on the cracking process which tends to form elemental carbon. That reaction, with the corresponding dissociation of the constituent atoms in water, is balanced against the twin formation processes creating carbon dioxide and hydrogen.
While cracking occurs rapidly, the recombination of the atomic constituents of steam and hydrocarbons into fuel does not. Carbon monoxide and hydrogen form slowly, limited by the rate-limiting reactions of the reformation process. Formation of carbon monoxide and hydrogen fuel also requires the majority of the energy of reformation. In addition, the steam constituent in the feedstock absorbs energy during dissociation. Also, the formation of carbon monoxide and hydrogen is not augmented so much by temperatures elevated above a minimum value as it is by dwell time. Access time is required to allow free atoms to find sites for recombination.
Of course, as a reaction begins at one end of a long reformer, the feedstock will be rich in unreformed hydrocarbons. Therefore, in light of the ease of cracking, too-high temperatures are particularly onerous, while site availability for recombination is low. Thus, the minimum temperature suitable for recombination should not be exceeded or the reformation and indeed the reformer may be overwhelmed by cracking.
Likewise, toward the end of a long reformer, the flow will be rich in fuel, and elevated temperatures may be helpful to increase the vigor of the reaction of the little remaining feedstock. Particularly since reaction sites are more widely dispersed, a high atomic collision rate is needed to drive the reaction rates of reformation. Likewise, with little feedstock remaining, carbonization of the catalyst in the reactor bed is less likely to be a problem.
An ideal reformer will maintain the temperature, heat flux and species concentrations at values which maintain the desired thermodynamic equilibrium. Thus, what is needed for an internal reformer is a system for distributing the temperature along the flow path of the feedstock in the reactor bed of a reformer. Heat transfer should be managed to provide adequate energy as heat flux to the endothermic processes at a temperature which is appropriate to the reaction kinetics, flow rate and dwell time. Such a system should prevent carbonization in feedstock-rich portions of the flow path. It should also ensure that reformation is as nearly complete as possible at the end of the flow path. That is, it should maximize reformation while minimizing carbonization due to excessive early cracking. It should match species concentrations, heat transfer and temperature throughout, yielding a smoothly declining feedstock-to-fuel ratio. Reaction sites for forming fuel will at first be plentiful, then dispersed, as the feedstock represents a decreasing fraction of the fluid stream in the reformer. Conversion rates approaching 100 percent should be achieved with no carbon build-up in reformers of commercially reasonable size.